1. Technical Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically to an effective technology for application to a liquid crystal display device employing an analog interface to which a gradation voltage is inputted from outside.
2. Description of Related Arts
Liquid crystal display devices are mainly classified into: those of a type to which display data (digital data) is inputted from outside; and those of a type (hereinafter, referred to as liquid crystal display devices for analog interface application) to which a gradation voltage (analog image voltage) is inputted from outside.
In the liquid crystal display device for analog interface application, gradation voltages are inputted from outside; therefore, a shift register circuit and a switching element for distributing the gradation voltages inputted from outside to respective image lines need to be provided on the liquid crystal display device side.
On the other hand, in an active-matrix-type liquid crystal display panel, each sub pixel has a thin-film transistor. As this thin-film transistor, those having a semiconductor layer formed of amorphous silicon and those having a semiconductor layer formed of polysilicon are well-known.
Hereinafter, a thin-film transistor having a semiconductor layer formed of amorphous silicon is referred to as an a-Si transistor, a liquid crystal display device using an a-Si transistor as an active element is referred to as an a-Si transistor, a thin-film transistor having a semiconductor layer formed of polysilicon is referred to as a p-Si transistor, and a liquid crystal display device using a p-Si transistor as an active element is referred to as a p-Si liquid crystal display device.
The aforementioned liquid crystal display device for analog interface application is required to sequentially sample analog gradation voltages inputted from outside to write them into respective sub pixels in one display line within one scanning period.
However, due to a small degree of movement of the a-Si transistor, in a case where the a-Si liquid crystal display device is used as a liquid crystal display device for analog interface application, analog gradation voltages inputted from outside cannot be sequentially sampled to be written into respective sub pixels in one display line within one scanning period.
Thus, as a conventional liquid crystal display device for analog interface application, the p-Si liquid crystal display device is used.
Further, in this p-Si liquid crystal display device, a shift register circuit and a switching element for distributing gradation voltages inputted from outside to respective image lines and also a scanning line driving circuit (shift register) for driving scanning lines are integrally formed on a substrate where the p-Si transistor is formed.
A liquid crystal display panel of the p-Si liquid crystal display device and a liquid crystal display panel of the a-Si liquid crystal display device are both fabricated by photolithography techniques. To fabricate the liquid crystal display panel of the p-Si liquid crystal display device, approximately ten masks are typically used.
Thus, the p-Si liquid crystal display device for analog interface application, suffers from difficulties in achieving cost reduction and poses a problem of deteriorated throughput.
Use of the a-Si liquid crystal display device as the liquid crystal display device for analog interface application permits achieving cost reduction and also improving throughput. However, as described above, due to a small degree of movement of the a-Si transistor, the a-Si liquid crystal display device fails to sequentially sample analog gradation voltages inputted from outside to write them into respective sub pixels in one display line within one scanning period.